Moshi’s OmniGuard™ series is a new collection of face and headwear to protect against bacteria, viruses, pollen, dust, smoke, pollution and more. The mask is water-resistant and features a long-lasting NanoShield™ antimicrobial coating for superior cleanliness and hygiene. Moshi's proprietary Nanohedron™ filter blocks particles as small as PM 0.075 (or 75 nanometers), offering 99% protection against the most commonly-found airborne contaminants. An anti-odor bamboo charcoal filter lines the inside of the mask for added comfort and wearability. Measure the distance between your lower eyelid and chin to determine the correct size. Small 3.4 - 4.1 in (8.6 - 10.5 cm), Medium 4.2 - 4.6 in (10.6 - 11.7 cm), Large Over 4.7 in (11.8 cm).